Power semiconductor modules are semiconductor apparatuses that are composed of a semiconductor device (1), a copper pattern (14), an insulating substrate (15), a conductor layer (16) and a heat dissipating plate (4) and they are extensively used in home electronic appliances such as air conditioners and washing machines, as well as in automobiles and industrial machines.
The recent trend of IC packages as in PCs is toward higher density and greater sophistication in functions; as a result, heat generation has become an important problem to deal with and the heat dissipating plate (4) called “a heat spreader” is currently used. Since the heat generated from power semiconductor modules and IC packages must be efficiently dissipated, the heat dissipating plate (4) is required to have high heat conductivity.
Other diverse characteristics are required of the heat dissipating plate (4). Take, for example, a power semiconductor module; in the assembling step, a metal-ceramics joined board (2) (hereunder referred to as “joined board (2)”) and the heat dissipating plate (4) are joined by solder (3) and the integrity of the solder (3) joint is important; also important is the flatness of the heat dissipating plate (4) since it is mounted on a heat sink.
During use, the power semiconductor module is subject to extensive temperature changes depending upon the operating situation and stress is exerted on the solder (3) joint between the joined board (2) and the heat dissipating plate (4) that have different thermal expansion coefficients. During such heat cycles, no defects such as cracking should occur in the solder (3) joint.
Speaking of the heat dissipating plate (4) that is to be used in IC packages, the reliability of the joint to the semiconductor chip is important; the heat dissipating plate (4) that is to be used in BGA packages and the like needs sufficient strength (rigidity) to serve as a back plate. Its strength (rigidity) should not drop in the assembling step.
Thus, the heat dissipating plate (4) to be used in power semiconductor modules and IC packages must satisfy diverse requirements including good heat conductivity; further, their prices are desirably low in view of the declining prices of home electronic appliances and PCs.
As the material (9) to be used in the heat dissipating plate (4), Cu—Cu2O, Al—SiC, Cu—W and other combined systems that are close to the insulating substrate (15) and IC chips in terms of thermal expansion coefficient may be considered but they are not cost-effective, nor do they have adequate heat conductivity.
Instead, copper-base alloys that have high heat conductivity and are also advantageous costwise are extensively used. Oxygen-free copper, which has high heat conductivity and is common as the material (9) for the heat dissipating plate (4), does not provide adequate 0.2% yield strength for the material (9) and hence is not capable of preventing the deformation of the heat dissipating plate (4) which is also required to work as a reinforcement. In addition, the heat dissipating plate (4) needs to be heated at 200–350° C. for several minutes in the joining step. If oxygen-free copper is heated under this condition, the material (9) will soften, making it difficult to achieve the desired flatness of the heat dissipating plate (4) after assembly.
Aside from oxygen-free copper, Cu—Zr, Cu—Ag, Cu—Sn, Cu—Sn—P and Cu—(Fe,Co,Ni)—P systems are practical copper-base alloys having high heat conductivity. However, the P-free Cu—Zr, Cu—Ag and Cu—Sn systems will have higher oxygen concentration if they are melted and solidified in the atmosphere during casting. To deal with this problem, a facility capable of atmosphere control is required but this causes a disadvantage in terms of production cost. The Cu—Zr and Cu—Ag systems are also disadvantageous in terms of the prices of their ingredients. Further, the Cu—Ag system is not satisfactory in 0.2% yield strength and heat-resisting characteristic; as for the Cu—Sn system, there occurs insufficiency in 0.2% yield strength and heat-resisting characteristic if the Sn concentration is low whereas there occurs a drop in heat conductivity if the Sn concentration is high. The Cu—Sn—P system has similar characteristics to the Cu—Sn system. Although conventionally not recognized as the material (9) for the heat dissipating plate (4), the Cu—(Fe,Co,Ni)—P system is a precipitation-strengthening type copper-base alloy and has good balance between physical properties. (0.2% yield strength and heat resistance) and electrical conductivity.
However, the above-mentioned copper-base alloys have thermal expansion coefficients of 16×10−6˜18 ×10−6/K whereas of all AlN, Al2O3, etc. which are used in the insulating substrate (15) of the power semiconductor module, and Si, etc. which are used in semiconductor chips have thermal expansion coefficients of less than 10×10−6/K; hence, in the case of using those copper-base alloys as the material (9) for the heat dissipating plate (4), the reliability of the joint during assembly has been a problem to address.
Consider, for example, the power semiconductor module; if the heat dissipating plate (4) is joined to the joined board (2) by means of solder (3), the former will warp due to the thermal expansion mismatch as the solder (3) solidifies (see FIG. 3). If such warped heat dissipating plate (4) is screwed to a heat sink, the area of contact is so small that the required heat dissipating quality cannot be obtained. If the heat sink and the heat dissipating plate (4) are joined by means of an increased number of screws with a view to increasing the area of their contact, cracking may occur in the solder (3) joint or the insulating substrate (15) may break. Therefore, the flatness of the heat dissipating plate (4) after assembly has been a problem to address.
The present invention has been accomplished in view of these problems and has as an object providing the heat dissipating plate (4) that is suitable for use in semiconductor apparatuses such as power semiconductor modules and IC packages and which is inexpensive, has high heat conductivity and assures a highly reliable joint during assembly and use.